One object of this invention is to produce an accurate image of a license plate while it is carried on a vehicle. License plates are normally illuminated for viewing by the sun, street lights, car lights and other light sources, and the vehicle carrying the plate is usually required by law to have lights that illuminate the plate at night. For human viewing and for many applications, this type of illumination is more than sufficient. However, it has been discovered that, in order to accurately produce an image of the license plate for more exacting applications such as image processing and/or recognition by computer, the normal ambient illumination should be considered as corrupting light. In other words, the normal illumination reflected by the license plate is usually considered to be a source of information, but it should be regarded as a source of noise.
Corrupting light (normal illumination) often impinges upon the license plate at an acute angle and will create shadows. For example, when the sun is overhead, the angle of incidence of sun light on the license plate is usually acute and if the license plate is recessed in the vehicle, a shadow from the car may be formed across the license plate. In some cases, the edge of the shadow may be in the center of the license plate. In such case, the license plate will include two areas that are receiving very different levels of light intensity. It is difficult to correct an image to compensate for these two different intensity levels and, of course, other sources of corrupting light may create similar shadows which add to the complexity of the shadow problem.
Many license plates are embossed and the embossing may also cause problems in the imaging process. When the characters of a license plate are embossed or raised, corrupting light may create shadows of the characters themselves on the license plate. Part of one character may fall in the shadow of the same character, or one character may shadow another character. In any event, the presence of shadows on the license plate make it difficult to accurately image the characters of the license plate. Also, the embossing creates numerous convex and concave surfaces on the license plate that will result in areas of high intensity and low intensity in terms of surface reflected light. Finally, the reflective properties of the vehicle on which the license plate is mounted will often create problems for imaging the license plate. For example, there may be areas of glare or high light intensity on the vehicle itself which will interfere with the equipment used to image the license plate.
The problems associated with imaging license plates have been recognized and are solved by the present invention in which normal illumination is treated as noise and is overcome. In one embodiment thereof, an optical system for producing an image of a set of characters on a background, such as a license plate, includes a flash light source for directing light toward the set of characters for a predetermined flash time period. A first polarizer is used to polarize the light from the flash light source and only the polarized light is transmitted to impinge upon the set of characters. The intensity of the illumination from the first polarizer exceeds the intensity of corrupting light from the environment of the license plate during the flash time period. A second polarizer is positioned to receive light that emanates from the set of characters and background, and the second polarizer is cross-polarized with respect to the first polarizer. In this configuration, the second polarizer is operable to filter out polarized light from the first polarizer that is specularly reflected by the set of characters and the background. Light from the first polarizer that is reflected and de-polarized by the set of characters and the background is partially transmitted through the second polarizer. An optical receiver is positioned behind the second polarizer and receives light from the set of characters and the background. This optical receiver produces an image of the set of characters and the background, and a shutter is provided to control the light that strikes the optical receiver. The shutter is synchronized with the optical receiver and flash light source so that it opens and closes during the flash period to admit light into the optical receiver. In this manner, the image produced by the optical receiver is dominantly produced by de-polarized flash light that is reflected by the set of characters and the background and is transmitted through the second polarizer and through the shutter into the optical receiver. In the preferred system, the second polarizer and shutter are the same device, a Kerr cell. However this is not necessary for the technique to work.
Since the shutter speed is open for a time that is less than or equal to the duration of the flash, and since the illumination from the flash source has an intensity greatly exceeding the intensity of the corrupting light and is synchronized with the opening of the shutter, the image produced by the optical receiver is dominantly from the light produced by the flash light source. In the preferred embodiment, the flash light source is one or more Xenon gas discharge type flashes. Also, the system may also include a constant source of light. The function of this constant light source is to mask the flash so that it will not be as noticeable.
Finally, in addition to synchronizing the shutter with the flash, the optical receiver is synchronized with either the flash or the shutter. In the preferred form, the optical receiver is a video camera that is synchronized to begin absorbing received light just prior to the opening of the shutter.